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Michael DeHaan 2012-05-13 12:03:24 -04:00
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<div class="section" id="best-practices">
<h1>Best Practices<a class="headerlink" href="#best-practices" title="Permalink to this headline"></a></h1>
<p>Here are some tips for making the most of Ansible.</p>
<div class="section" id="group-by-roles">
<h2>Group By Roles<a class="headerlink" href="#group-by-roles" title="Permalink to this headline"></a></h2>
<p>A system can be in multiple groups. See ref:<cite>patterns</cite>. Having groups named after things like
&#8216;webservers&#8217; and &#8216;dbservers&#8217; is repeated in the examples because it&#8217;s a very powerful concept.</p>
<p>This allows playbooks to target machines based on role, as well as to assign role specific variables
using the group variable system.</p>
</div>
<div class="section" id="directory-organization">
<h2>Directory Organization<a class="headerlink" href="#directory-organization" title="Permalink to this headline"></a></h2>
<p>Playbooks should be organized like this:</p>
<div class="highlight-python"><pre>(root of source control repository)
acme/ # each playbook has a directory
setup.yml # playbook to manage the service
stop.yml # playbook to halt the service (optional)
files/
some_file_path_foo.conf
templates/
etc_acme_conf_acme.conf
etc_other_conf_other.conf
vars/
main.yml
handlers/
main.yml
tasks/
setup.yml
stop.yml</pre>
</div>
<p>Any directories or files not needed can be omitted. Not all modules may require <cite>vars</cite> or <cite>files</cite> sections, though most
will require <cite>handlers</cite>, <cite>tasks</cite>, and <cite>templates</cite>. To review what each of these sections do, see ref:<cite>playbooks</cite> and ref:<cite>playbooks2</cite>.</p>
<p>The acme/setup.yml playbook would be as simple as:</p>
<div class="highlight-python"><pre>----
- hosts: webservers
user: root
vars_files
- include: vars/main.yml
tasks:
- include: tasks/setup.yml
handlers:
- include: handlers/main.yml</pre>
</div>
<p>The tasks are individually broken out in &#8216;acme/tasks/setup.yml&#8217;, and handlers, which are common to all task files,
are contained in &#8216;acme/handlers/main.yml&#8217;. As a reminder, handlers are mostly just used to notify services to restart
when things change, and these are described in ref:<cite>playbooks</cite>.</p>
<p>Including more than one setup file or more than one handlers file is of course legal.</p>
<p>Having playbooks be able to include other playbooks is coming in release 0.5.</p>
<p>Until then, to manage your entire site, simply execute all of your playbooks together, in the order desired.
You don&#8217;t have to do this though, it&#8217;s fine to select sections of your infrastructure to manage at a single time.
You may wish to construct simple shell scripts to wrap calls to ansible-playbook.</p>
</div>
<div class="section" id="miscellaneous-tips">
<h2>Miscellaneous Tips<a class="headerlink" href="#miscellaneous-tips" title="Permalink to this headline"></a></h2>
<p>When you can do something simply, do something simply. Do not reach to use every feature of Ansible together, all
at once. Use what works for you. For example, you should probably not need &#8216;vars&#8217;, &#8216;vars_files&#8217;, &#8216;vars_prompt&#8217; and &#8216;&#8211;extra-vars&#8217; all at once, while also using an external inventory file.</p>
<p>Optimize for readability. Whitespace between sections of YAML documents and in between tasks is strongly encouraged,
as is usage of YAML comments, which start with &#8220;#&#8221;. It is also useful to comment at the top of each file the purpose of the individual file and the author, including email address.</p>
<p>It is possible to leave off the &#8220;name&#8221; for a given task, though it is recommended to provide
a descriptive comment about why something is being done instead.</p>
<p>Use version control. Keep your playbooks and inventory file in git (or another version control system), and commit when you make changes to them.
This way you have an audit trail describing when and why you changed the rules automating your infrastructure.</p>
<p>Resist the urge to write the same playbooks and configuration files for heterogeneous distributions. While lots of software packages claim to make this easy on you, the configuration files are often quite different, to the point where it would be easier to treat them as different playbooks. This is why, for example, Ansible has a seperate &#8216;yum&#8217; and &#8216;apt&#8217; module. Yum and apt have different capabilities, and we don&#8217;t want to code for the least common denominator.</p>
<p>Use variables for user tunable settings versus having constants in the tasks file or templates, so that it is easy to reconfigure a playbook. Think about this as exposing the knobs to things you would like to tweak.</p>
<p>Since a system can be in more than one group, if you have multiple datacenters or sites, consider putting systems into groups by role, but also different groups by geography. This allows you to assign different variables to different geographies.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference internal" href="YAMLSyntax.html"><em>YAML Syntax</em></a></dt>
<dd>Learn about YAML syntax</dd>
<dt><a class="reference internal" href="playbooks.html"><em>Playbooks</em></a></dt>
<dd>Review the basic playbook features</dd>
<dt><a class="reference internal" href="modules.html"><em>Ansible Modules</em></a></dt>
<dd>Learn about available modules</dd>
<dt><a class="reference internal" href="moduledev.html"><em>Module Development</em></a></dt>
<dd>Learn how to extend Ansible by writing your own modules</dd>
<dt><a class="reference internal" href="patterns.html"><em>Inventory &amp; Patterns</em></a></dt>
<dd>Learn about how to select hosts</dd>
<dt><a class="reference external" href="https://github.com/ansible/ansible/tree/master/examples/playbooks">Github examples directory</a></dt>
<dd>Complete playbook files from the github project source</dd>
<dt><a class="reference external" href="http://groups.google.com/group/ansible-project">Mailing List</a></dt>
<dd>Questions? Help? Ideas? Stop by the list on Google Groups</dd>
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<li><a class="reference internal" href="#variable-file-seperation">Variable File Seperation</a></li>
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<div class="section" id="advanced-playbooks">
<h1>Advanced Playbooks<a class="headerlink" href="#advanced-playbooks" title="Permalink to this headline"></a></h1>
<p>Here are some advanced features of the playbooks language. Using all of these features
are not neccessary, but many of them will prove useful. If a feature doesn&#8217;t seem immediately
relevant, feel free to skip it. For many people, the features documented in <cite>playbooks</cite> will
be 90% or more of what they use in Ansible.</p>
<div class="section" id="accessing-complex-variable-data">
<h2>Accessing Complex Variable Data<a class="headerlink" href="#accessing-complex-variable-data" title="Permalink to this headline"></a></h2>
<p>Some provided facts, like networking information, are made available as nested datastructures. To access
them a simple &#8216;$foo&#8217; is not sufficient, but it is still easy to do. Here&#8217;s how we get an IP address using
Ansible 0.4 and later:</p>
<div class="highlight-python"><pre>${ansible_eth0.ipv4.address}</pre>
</div>
<p>It is also possible to access variables whose elements are arrays:</p>
<div class="highlight-python"><pre>${somelist[0]}</pre>
</div>
<p>And the array and hash reference syntaxes can be mixed.</p>
<p>In templates, the simple access form still holds, but they can also be accessed from Jinja2 in more Python-native ways if
that is preferred:</p>
<div class="highlight-python"><pre>{{ ansible_eth0["ipv4"]["address"] }}</pre>
</div>
</div>
<div class="section" id="accessing-information-about-other-hosts">
<h2>Accessing Information About Other Hosts<a class="headerlink" href="#accessing-information-about-other-hosts" title="Permalink to this headline"></a></h2>
<p>If your database server wants to check the value of a &#8216;fact&#8217; from another node, or an inventory variable
assigned to another node, it&#8217;s easy to do so within a template or even an action line (note: this uses syntax available in 0.4 and later):</p>
<div class="highlight-python"><pre>${hostvars.hostname.factname}</pre>
</div>
<p>NOTE: No database or other complex system is required to exchange data between hosts. The hosts that you
want to reference data from must be included in either the current play or any previous play.</p>
</div>
<div class="section" id="magic-variables">
<h2>Magic Variables<a class="headerlink" href="#magic-variables" title="Permalink to this headline"></a></h2>
<p>Some variables made available to hosts don&#8217;t come from definitions in a playbook, the inventory file, or discovery from the system. There are only two of these, and are used in special cases that many users won&#8217;t need.</p>
<p><cite>groups</cite> is a list (array) of all the groups the current host is in. This can be used in templates using Jinja2
syntax to make template source files that vary based on the group membership (or role) of the host:</p>
<div class="highlight-python"><pre>{% if 'webserver' in groups %}
# some part of a configuration file that only applies to webservers
{% endif %}</pre>
</div>
<p><cite>inventory_hostname</cite> is the name of the hostname as configured in Ansible&#8217;s inventory host file. This can
be useful for when you don&#8217;t want to rely on the discovered hostname <cite>ansible_hostname</cite> or for other mysterious
reasons. Don&#8217;t worry about it unless you think you need it.</p>
</div>
<div class="section" id="variable-file-seperation">
<h2>Variable File Seperation<a class="headerlink" href="#variable-file-seperation" title="Permalink to this headline"></a></h2>
<p>It&#8217;s a great idea to keep your playbooks under source control, but
you may wish to make the playbook source public while keeping certain
important variables private. Similarly, sometimes you may just
want to keep certain information in different files, away from
the main playbook.</p>
<p>You can do this by using an external variables file, or files, just like this:</p>
<div class="highlight-python"><pre>---
- hosts: all
user: root
vars:
favcolor: blue
vars_files:
- /vars/external_vars.yml
tasks:
- name: this is just a placeholder
action: command /bin/echo foo</pre>
</div>
<p>This removes the risk of sharing sensitive data with others when
sharing your playbook source with them.</p>
<p>The contents of each variables file is a simple YAML dictionary, like this:</p>
<div class="highlight-python"><pre>---
# in the above example, this would be vars/external_vars.yml
somevar: somevalue
password: magic</pre>
</div>
</div>
<div class="section" id="prompting-for-sensitive-data">
<h2>Prompting For Sensitive Data<a class="headerlink" href="#prompting-for-sensitive-data" title="Permalink to this headline"></a></h2>
<p>You may wish to prompt the user for certain input, and can
do so with the similarly named &#8216;vars_prompt&#8217; section. This has uses
beyond security, for instance, you may use the same playbook for all
software releases and would prompt for a particular release version
in a push-script:</p>
<div class="highlight-python"><pre>---
- hosts: all
user: root
vars:
from: "camelot"
vars_prompt:
name: "what is your name?"
quest: "what is your quest?"
favcolor: "what is your favorite color?"</pre>
</div>
<p>There are full examples of both of these items in the github examples/playbooks directory.</p>
</div>
<div class="section" id="passing-variables-on-the-command-line">
<h2>Passing Variables On The Command Line<a class="headerlink" href="#passing-variables-on-the-command-line" title="Permalink to this headline"></a></h2>
<p>In addition to <cite>vars_prompt</cite> and <cite>vars_files</cite>, it is possible to send variables over
the ansible command line. This is particularly useful when writing a generic release playbook
where you may want to pass in the version of the application to deploy:</p>
<div class="highlight-python"><pre>ansible-playbook release.yml --extra-vars "version=1.23.45 other_variable=foo"</pre>
</div>
</div>
<div class="section" id="conditional-execution">
<h2>Conditional Execution<a class="headerlink" href="#conditional-execution" title="Permalink to this headline"></a></h2>
<p>Sometimes you will want to skip a particular step on a particular host. This could be something
as simple as not installing a certain package if the operating system is a particular version,
or it could be something like performing some cleanup steps if a filesystem is getting full.</p>
<p>This is easy to do in Ansible, with the <cite>only_if</cite> clause, which actually is a Python expression.
Don&#8217;t panic &#8211; it&#8217;s actually pretty simple:</p>
<div class="highlight-python"><pre>vars:
favcolor: blue
is_favcolor_blue: "'$favcolor' == 'blue'"
is_centos: "'$facter_operatingsystem' == 'CentOS'"
tasks:
- name: "shutdown if my favorite color is blue"
action: command /sbin/shutdown -t now
only_if: '$is_favcolor_blue'</pre>
</div>
<p>Variables from tools like <cite>facter</cite> and <cite>ohai</cite> can be used here, if installed, or you can
use variables that bubble up from ansible (0.3 and later). As a reminder,
these variables are prefixed, so it&#8217;s <cite>$facter_operatingsystem</cite>, not <cite>$operatingsystem</cite>. Ansible&#8217;s
built in variables are prefixed with <cite>ansible_</cite>. The only_if
expression is actually a tiny small bit of Python, so be sure to quote variables and make something
that evaluates to <cite>True</cite> or <cite>False</cite>. It is a good idea to use &#8216;vars_files&#8217; instead of &#8216;vars&#8217; to define
all of your conditional expressions in a way that makes them very easy to reuse between plays
and playbooks.</p>
</div>
<div class="section" id="conditional-imports">
<h2>Conditional Imports<a class="headerlink" href="#conditional-imports" title="Permalink to this headline"></a></h2>
<p>Sometimes you will want to do certain things differently in a playbook based on certain criteria.
Having one playbook that works on multiple platforms and OS versions is a good example.</p>
<p>As an example, the name of the Apache package may be different between CentOS and Debian,
but it is easily handled with a minimum of syntax in an Ansible Playbook:</p>
<div class="highlight-python"><pre>---
- hosts: all
user: root
vars_files:
- "vars/common.yml"
- [ "vars/$facter_operatingsystem.yml", "vars/os_defaults.yml" ]
tasks:
- name: make sure apache is running
action: service name=$apache state=running</pre>
</div>
<p>Note that a variable (<cite>$facter_operatingsystem</cite>) is being interpolated into the list of
filenames being defined for vars_files.</p>
<p>As a reminder, the various YAML files contain just keys and values:</p>
<div class="highlight-python"><pre>---
# for vars/CentOS.yml
apache: httpd
somethingelse: 42</pre>
</div>
<p>How does this work? If the operating system was &#8216;CentOS&#8217;, the first file Ansible would try to import
would be &#8216;vars/CentOS.yml&#8217;, followed up by &#8216;/vars/os_defaults.yml&#8217; if that file
did not exist. If no files in the list were found, an error would be raised.
On Debian, it would instead first look towards &#8216;vars/Debian.yml&#8217; instead of &#8216;vars/CentOS.yml&#8217;, before
falling back on &#8216;vars/os_defaults.yml&#8217;. Pretty simple.</p>
<p>To use this conditional import feature, you&#8217;ll need facter or ohai installed prior to running the playbook, but
you can of course push this out with Ansible if you like:</p>
<div class="highlight-python"><pre># for facter
ansible -m yum -a "pkg=facter ensure=installed"
ansible -m yum -a "pkg=ruby-json ensure=installed"
# for ohai
ansible -m yum -a "pkg=ohai ensure=installed"</pre>
</div>
<p>Ansible&#8217;s approach to configuration &#8211; seperating variables from tasks, keeps your playbooks
from turning into arbitrary code with ugly nested ifs, conditionals, and so on - and results
in more streamlined &amp; auditable configuration rules &#8211; especially because there are a
minimum of decision points to track.</p>
</div>
<div class="section" id="loop-shorthand">
<h2>Loop Shorthand<a class="headerlink" href="#loop-shorthand" title="Permalink to this headline"></a></h2>
<p>To save some typing, repeated tasks can be written in short-hand like so:</p>
<div class="highlight-python"><pre>- name: add user $item
action: user name=$item state=present groups=wheel
with_items:
- testuser1
- testuser2</pre>
</div>
<p>The above would be the equivalent of:</p>
<div class="highlight-python"><pre>- name: add user testuser1
action: user name=testuser1 state=present groups=wheel
- name: add user testuser2
action: user name=testuser2 state=present groups=wheel</pre>
</div>
<p>In a future release, the yum and apt modules will use with_items to execute fewer package
manager transactions.</p>
</div>
<div class="section" id="selecting-files-and-templates-based-on-variables">
<h2>Selecting Files And Templates Based On Variables<a class="headerlink" href="#selecting-files-and-templates-based-on-variables" title="Permalink to this headline"></a></h2>
<p>Sometimes a configuration file you want to copy, or a template you will use may depend on a variable.
The following construct (new in 0.4) selects the first available file appropriate for the variables of a given host,
which is often much cleaner than putting a lot of if conditionals in a template.</p>
<p>The following example shows how to template out a configuration file that was very different between, say,
CentOS and Debian:</p>
<div class="highlight-python"><pre>- name: template a file
action: template src=$item dest=/etc/myapp/foo.conf
first_available_file:
- /srv/templates/myapp/${ansible_distribution}.conf
- /srv/templates/myapp/default.conf</pre>
</div>
</div>
<div class="section" id="asynchronous-actions-and-polling">
<h2>Asynchronous Actions and Polling<a class="headerlink" href="#asynchronous-actions-and-polling" title="Permalink to this headline"></a></h2>
<p>By default tasks in playbooks block, meaning the connections stay open
until the task is done on each node. If executing playbooks with
a small parallelism value (aka <tt class="docutils literal"><span class="pre">--forks</span></tt>), you may wish that long
running operations can go faster. The easiest way to do this is
to kick them off all at once and then poll until they are done.</p>
<p>You will also want to use asynchronous mode on very long running
operations that might be subject to timeout.</p>
<p>To launch a task asynchronously, specify its maximum runtime
and how frequently you would like to poll for status. The default
poll value is 10 seconds if you do not specify a value for <cite>poll</cite>:</p>
<div class="highlight-python"><pre>---
- hosts: all
user: root
tasks:
- name: simulate long running op (15 sec), wait for up to 45, poll every 5
action: command /bin/sleep 15
async: 45
poll: 5</pre>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">There is no default for the async time limit. If you leave off the
&#8216;async&#8217; keyword, the task runs synchronously, which is Ansible&#8217;s
default.</p>
</div>
<p>Alternatively, if you do not need to wait on the task to complete, you may
&#8220;fire and forget&#8221; by specifying a poll value of 0:</p>
<div class="highlight-python"><pre>---
- hosts: all
user: root
tasks:
- name: simulate long running op, allow to run for 45, fire and forget
action: command /bin/sleep 15
async: 45
poll: 0</pre>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">You shouldn&#8217;t &#8220;fire and forget&#8221; with operations that require
exclusive locks, such as yum transactions, if you expect to run other
commands later in the playbook against those same resources.</p>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Using a higher value for <tt class="docutils literal"><span class="pre">--forks</span></tt> will result in kicking off asynchronous
tasks even faster. This also increases the efficiency of polling.</p>
</div>
</div>
<div class="section" id="local-playbooks">
<h2>Local Playbooks<a class="headerlink" href="#local-playbooks" title="Permalink to this headline"></a></h2>
<p>It may be useful to use a playbook locally, rather than by connecting over SSH. This can be useful
for assuring the configuration of a system by putting a playbook on a crontab. This may also be used
to run a playbook inside a OS installer, such as an Anaconda kickstart.</p>
<p>To run an entire playbook locally, just set the &#8220;hosts:&#8221; line to &#8220;hosts:127.0.0.1&#8221; and then run the playbook like so:</p>
<div class="highlight-python"><pre>ansible-playbook playbook.yml --connection=local</pre>
</div>
<p>Alternatively, a local connection can be used in a single playbook play, even if other plays in the playbook
use the default remote connection type:</p>
<div class="highlight-python"><pre>hosts: 127.0.0.1
connection: local</pre>
</div>
</div>
<div class="section" id="pull-mode-playbooks">
<h2>Pull-Mode Playbooks<a class="headerlink" href="#pull-mode-playbooks" title="Permalink to this headline"></a></h2>
<p>The use of playbooks in local mode (above) is made extremely powerful with the addition of <cite>ansible-pull</cite> in the
0.4 release. A script for setting up ansible-pull is provided in the examples/playbooks directory of the source
checkout.</p>
<p>The basic idea is to use Ansible to set up a remote copy of ansible on each managed node, each set to run via
cron and update playbook source via git. This interverts the default push architecture of ansible into a pull
architecture, which has near-limitless scaling potential. The setup playbook can be tuned to change
the cron frequency, logging locations, and parameters to ansible-pull.</p>
<p>This is useful both for extreme scale-out as well as periodic remediation. Usage of the &#8216;fetch&#8217; module to retrieve
logs from ansible-pull runs would be an excellent way to gather and analyze remote logs from ansible-pull.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference internal" href="YAMLSyntax.html"><em>YAML Syntax</em></a></dt>
<dd>Learn about YAML syntax</dd>
<dt><a class="reference internal" href="playbooks.html"><em>Playbooks</em></a></dt>
<dd>Review the basic playbook features</dd>
<dt><a class="reference internal" href="bestpractices.html"><em>Best Practices</em></a></dt>
<dd>Various tips about playbooks in the real world</dd>
<dt><a class="reference internal" href="modules.html"><em>Ansible Modules</em></a></dt>
<dd>Learn about available modules</dd>
<dt><a class="reference internal" href="moduledev.html"><em>Module Development</em></a></dt>
<dd>Learn how to extend Ansible by writing your own modules</dd>
<dt><a class="reference internal" href="patterns.html"><em>Inventory &amp; Patterns</em></a></dt>
<dd>Learn about how to select hosts</dd>
<dt><a class="reference external" href="https://github.com/ansible/ansible/tree/master/examples/playbooks">Github examples directory</a></dt>
<dd>Complete playbook files from the github project source</dd>
<dt><a class="reference external" href="http://groups.google.com/group/ansible-project">Mailing List</a></dt>
<dd>Questions? Help? Ideas? Stop by the list on Google Groups</dd>
</dl>
</div>
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Last updated on May 13, 2012.<br/>
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